The present invention concerns a telecommunications system providing high data bit rates and minimum propagation times.
Systems including a network of satellites in low Earth orbit, at an altitude in the range from 1000 km to 1500 km, for example, have already been proposed for providing high bit rate communication between terrestrial mobiles or stations in a particular geographical area covering several hundred kilometers. In such systems, each satellite includes groups of receive and transmit antennas and each group is dedicated to a given area. Within each group, the receive antennas receive the signals from a station in the area and the transmit antennas relay the received signals to another station in the same area. As the satellite moves, the antennas of a group point towards the area at all times so long as the area remains within the field of view of the satellite. Accordingly, for each satellite, a region of the Earth is divided into n areas, and when the satellite moves over a region, a group of transmit and receive antennas is allocated to each area and points toward that area at all times.
In this way, switching from one antenna to another while the satellite is moving over a region, which takes around twenty minutes, for example, which could be prejudicial to the speed or the quality of communication, is avoided because only one group of transmit and receive antennas is allocated to the area.
Furthermore, the low altitude of the satellites minimizes propagation times, which is favorable to interactive communications, especially for xe2x80x9cmultimediaxe2x80x9d applications.
The invention aims to optimize the equipment onboard the satellites and in the various areas.
To optimize the telecommunications system, the invention proposes to separate the areas into at least two categories. The first category comprises areas in which the demand for communication, or traffic, is below a threshold, and the second category comprises areas in which the demand is greater than or equal to the threshold. The respective polarizations and carrier frequencies of signals received and transmitted in each area of the first category (low traffic) are chosen so that signals having different polarizations or different carrier frequencies are allocated to two adjacent areas. Signals having all available polarizations and all available carrier frequencies are allocated to each area of the second category (high traffic).
In this way limited resources are allocated to areas of the first category and all the signal resources are allocated to areas of the second category.
The polarization is either circular or linear.
The above approach matches the equipment used to what is required. In particular, equipment for low traffic areas can be of simple design. The constraints imposed on the transmit and receive antennas on a satellite or a spacecraft which are allocated to low traffic areas are then less severe than the constraints imposed on the antennas for high traffic areas.
It has been found that the high traffic areas in each region amount to only approximately 25% of the total number of areas, although they account for from 40% to 50% of the total traffic.
The satellite transmit and receive antennas allocated to low traffic areas are active antennas, for example, i.e. antennas whose radiation patterns are formed and pointed entirely by electronic means. The shape of the pattern is also modified by electronic means. Note that modifying the shape of the pattern, which is specific to the field of telecommunications, is necessary because the shape of the radiation transmitted towards the ground varies with the position of the satellite relative to the area. For example, if the area to be covered on the ground is circular, the satellite at the nadir of that area xe2x80x9cseesxe2x80x9d a circular area. In contrast, as the satellite moves away from this position it sees the area as an ellipse.
Mechanically pointed antennas are used for high traffic areas, e.g. in association with preferably-electronic means for modifying the radiation pattern as a function of orientation, i.e. for matching the radiation pattern to the shape of the area xe2x80x9cseenxe2x80x9d by the antenna.